I. Field of the Invention
This invention relates to a process for the production of a high density jet fuel from a feed comprised of fused multi-ring aromatic hydrocarbons, or fused multi-ring hydroaromatic hydrocarbons, or both. In particular, it relates to the production of high density jet fuels from feedstocks containing large concentrations of fused two-ring aromatic or hydroaromatic compounds, or both, especially a hydrocarbon feed fraction boiling within a range of about 300.degree. F. to 550.degree. F., and more particularly from about 300.degree. F. to 525.degree. F.
II. Background and Problems
The preponderance of fused multi-ring aromatic or hydroaromatic hydrocarbons, or hydrocarbons which contain two or more fused aromatic or hydroaromatic rings, are found in mid-distillate fuel fractions produced in the refining of petroleum, or synthetic fuel fractions generally analogous thereto which are obtained from coal liquids, tar sands, shale oils and like materials. These compounds are substantially excluded from motor gasoline inasmuch as they produce carbonaceous deposits when burned in internal combustion engines. In burning, the fused multi-ring aromatics are directly converted to carbon, and the fused multi-ring hydroaromatics are dehydrogenated back into their corresponding parent fused multi-ring aromatic hydocarbons, respectively, and then converted to carbon; the same end result. The latter reaction is particularly favored at high temperatures such as prevails in internal combustion engines.
The presence of a small fraction of the fused two-ring aromatic or fused two-ring hydroaromatic hydrocarbons in gasolines, however, is often tolerated, since these compounds generally provide acceptable octane, and it is difficult to remove all of these compounds from the gasolines with any degree of economy. Their presence in gasolines, however, is costly in that their presence results in increased carbon deposition in internal combustion engines. The two-ring hydroaromatic hydrocarbon and higher boiling fused multi-ring aromatic and hydroaromatic hydrocarbons which contain three or more fused rings (sometimes termed PNA's for polynuclear aromatic hydrocarbons), obtained from refinery process streams are often blended into distillate fuels even though their cetane values are low, or further processed to make gasoline. In the former case, e.g., a light cat cycle oil, the product of a catalytic cracker rich in fused two-ring hydrocarbons, is often blended with high quality kerosene to upgrade the former and produce diesel fuel. In the latter case, a light cat cycle oil, boiling between about 400.degree. F. and 750.degree. F. is hydrotreated, and the product of the hydrotreating is hydrocracked to coproduce predominately naphtha and some jet fuel and diesel fuel.
In the coproduction of jet and diesel fuels, with gasoline, typically a light cat cycle oil, boiling within a range of from about 400.degree. F. to 750.degree. F., is hydrotreated in a first reactor over a catalyst containing a Group VIB or Group VIII metal, or both, to remove a preponderance of the sulfur and nitrogen. The liquid product from the hydrotreater is then hydrocracked to lower boiling or lower molecular weight hydrocarbons. Hydrocracking is a high severity hydrotreating operation employing an acidic base catalyst such as a faujasite or silica alumina containing Group VIII metal or Group VIB and combinations thereof to produce naphtha as the primary product. The sulfur and nitrogen thus are generally removed from the product as H.sub.2 S and NH.sub.3 by passage to a flash drum, or the total sour gas and product, is then conducted to a hydrocracker and reacted at high severity over a hydrocracking catalyst, e.g., a palladium-on-zeolite Y or nickel on silica-alumina base. Typically the hydrcracking reaction is conducted at temperatures ranging about 600.degree. F. to 750.degree. F. and 500 to 1500 pounds per square inch pressure (psig) to produce a major quantity of the product as naphtha, and a lesser quantity of the product as JP-1A or JP-5 jet fuel, and diesel blend components. These jet fuels are high flash point distillates of the kerosene type.
The heating value of aviation turbine, or jet fuels, measured by their heat of combustion, directly influences the flying range and payload capacity of all aircraft. Heating value is governed by the chemical composition of the fuel, and changes in chemical composition, notably the relative concentrations of the aromatics and paraffins, may raise or lower the heat of combustion depending on the weight or value of each, respectively, of the fuel. The effectivness of a jet fuel depends primarily on the amount of heat energy released when it is burned. The fuel capacity of an airplane is limited by the weight, or volume, of the fuel it can carry to advantage. In jet turbine engines, notably in high flying supersonic aircraft, a high proportion of the gross load is fuel, and therefore in such aircraft the composition of the fuel can be particularly important. The amount of fuel which an operational aircraft can carry is volume limited, and hence the density of a fluid can be particularly important since a fuel containing greater BTU's per unit can be contained within a given volume by increasing the density of a fuel. For example, it has been suggested that an increase in the heat of combustion from 18,400 BTU/lb. to 18,900 BTU/lb. allows a 30 percent increase in payload. Hence, a higher density fuel can increase the payload, or distance which a volume limited operational aircraft can fly, by utilizing a fuel containing more BTU's per gallon within its fuel tanks.
There presently exits a need for higher density jet fuels, particularly for use in aircraft which are volume limited in the amount of fuel which can be advantageously carried. In particular, there exists need for a process which can commercially produce from relatively low value feedstocks higher density aviation fuels, especially fuels for use in high flying supersonic aircraft which are volume limited in the amount of fuel which can be carried to advantage.